Liquid dispensing system with improved sealing augering screw and method for dispensing

ABSTRACT

A liquid dispenser has an augering screw that serves both a metering function and a valving function. The augering screw is axially movable between a position in which fluid can flow and a sealing position in which liquid flow through the nozzle is substantially prevented. The screw is preferably designed with a curved contour between the threads and with a plurality of thread-defining channels to allow liquid to fill more completely around the screw and to dispense more liquid with fewer screw turns than prior screw designs.

This application is a continuation application under 37 CFR §1.53(b) ofSer. No. 09/075,604 filed May 11, 1998, which is a continuationapplication under 37 CFR §1.53(b) of Ser. No. 08/562,068, filed Nov. 22,1995, now U.S. Pat. No. 5,819,983 incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a system and method for dispensing meteredamounts of viscous liquid onto a medium.

BACKGROUND OF THE INVENTION

Different types of machines are used for dispensing small meteredamounts of liquid for a variety of applications. In the assembly ofsurface mount printed circuit boards, one application is for dispensingmany small dots of adhesive liquid on a circuit board for connectingcomponents; another is for dispensing material over an area forencapsulating chips and/or for underfilling flip chips. Such dispensingmachines are expected to run continuously to achieve high throughput,and are also expected to have a high degree of repeatability, i.e., tobe able to dispense dots or areas with the same size within a very smalltolerance.

Some systems for dispensing dots, including earlier designs, used burstsof high pressure on a container of liquid and had a separate valve forcontrolling flow. This type of system was improved upon with a systemthat used a positive rotary displacement pump instead of bursts of highpressure. In one model manufactured by Knight Tool Co., the assignee ofthe present invention, and distributed by CAM/ALOT under the nameCamelot®, an augering screw is housed in an augering chamber and isrotated to provide a carefully metered amount of liquid. A motor iscoupled to the screw with a controllable electromagnetically operatedclutch. The clutch has a top plate that is continuously rotated by themotor, and a bottom plate that is rotatably coupled to the augeringscrew through intermediate coupling members, including a metal bellows.The liquid to be dispensed is held in a cylindrical container, and isprovided to the augering chamber under constant pressure of about 10PSI, a pressure that is considered rather low.

To dispense liquid, a controller provides to the clutch 5 a short,timed, electrical signal that induces magnetic attraction between thetop and bottom plates; This attraction causes the plates to be engagedand to rotate together for a short period of time, thus causing theaugering screw to rotate to dispense a small amount of liquid from theaugering chamber through a nozzle. The controller also controls leadscrew motors for moving the dispenser to a desired location along threemutually orthogonal axes.

Other systems that use a positive rotary displacement is pump drive thepump with a stepper motor that has to be turned on and off frequently.Such motors, however, must be sufficiently durable so that they canstart and stop many times without failure.

Dispensing system of this general type can be used to 20 dispense anarea of highly viscous liquid to cover the top of a semiconductor devicefor encapsulation, or to provide underfilling around and under a flipchip to provide thermal conductivity. Then the liquid hardens over oraround the device, it packages and protects the device. To cover thearea, the dispenser can be moved in a selected pattern, such as arastering pattern or a spiral pattern.

In such dispensing systems, the accuracy of the volume dispensed iscritically important. The material used for encapsulation and underfillis filled with abrasive particles and has a very high viscosity,typically from 10⁵ to 10⁶ centipoise. Consequently, pressure of at leastabout 30 to 40 PSI is provided to the container that holds the liquid toovercome friction in the container and to dispense the liquid. If thepressure is insufficient to properly feed the augering screw, cavitiesof air can develop in the liquid, adversely affecting accuracy (thisproblem is known as “cavitation”). Because this pressure is relativelyhigh compared to the dot dispensing systems, however, the liquid canbleed through the augering chamber and leak through the nozzle betweendispensing cycles. Such leaked liquid can have a substantial detrimentaleffect on the accuracy.

To prevent such bleeding and leaking, some models of dispensers have adirect drive stepper motor with a reversible drive for drawing theliquid back after a dispensing cycle is completed. Reversing a motorquickly 10 and frequently can adversely affect wear on the motor,however, and can also adversely affect accuracy.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to improve the is accuracy of aliquid dispensing system.

Another object of the present invention is to provide a reliable liquiddispensing system that does not leak between dispensing cycles.

Still another object is to provide a liquid dispensing 20 system withparts that can withstand repeated use and thus last for a long time.

Yet another object is to provide a dispenser that meets these otherobjects with a compact assembly.

The present invention includes a liquid dispensing 25 system and amethod for dispensing while substantially preventing undesired leakagethrough an outlet of a nozzle between dispensing cycles. The dispenserhas a cartridge with a housing that encloses an augering screw and anaugering chamber. The chamber receives a liquid input, and the augeringscrew augers the liquid from the chanter, through an outlet in thenozzle, and onto a medium. The augering screw also serves as a valvebetween the augering chamber and the outlet of the nozzle. The augeringscrew is controllably moved so that one end moves in and out of a valveseat between the augering chamber and the outlet of the nozzle. Thevalve seat can be formed in the nozzle itself, or between the augeringchamber and the nozzle. When the screw is in the valve seat, the seal issufficient to substantially prevent the liquid from flowing past theseal.

The augering screw preferably has an improved design in its threadingand shape, including a curved contour along the axial direction betweenthreads. This contour reduces air spaces around the screw and increasesthe percentage of the area of the screw that is in contact with theliquid.

Because of the improved amount of contact area, there is lessopportunity for air pockets to form around the screw, thus reducingchanges in pressure that can otherwise cause leaking. To increase theamount of fluid that is dispensed per rotation, the threads of the screware formed with a plurality of helical channels, preferably two channels180 apart about the screw. The channels are formed so that the improvedscrew has double the number of threads per inch compared to known priorscrews.

In one preferred embodiment, the augering screw is 20 formed integrallyin a drive shaft so that the drive shaft and screw move together axiallyand rotatably. An annular clip is rigidly connected to the drive shaftand a piston is disposed around the drive shaft and under the clip sothat it abuts the clip. The piston is biased downwardly with a springbut can receive upwardly directed gas pressure, preferably air pressure,thus causing the piston to be raised against the clip so that the screwis raised away from the valve seat. Then the air pressure stops, thescrew is biased downwardly by a second spring so that it rests in thevalve seat. The spring can be replaced with a gas inlet so that thepiston is both raised and lowered through the use of gas pressure. Amanually accessible, adjustable, threaded micrometer can be providedover the piston to control spacing between the retainer clip and theshaft. This micrometer allows the user to adjustably control the heightby which the shaft and screw are raised, preferably over a range ofabout 0-0.1 inches.

In another embodiment, the screw includes an annular nut around itslower end. The nut and the housing around the augering chamber define anannular valve seat. The augering screw is usually biased downwardly in aposition such that liquid can flow. To shut off the flow of liquid, thescrew is raised. By raising the screw to prevent flow, a slight suctionis created, thus further preventing leakage. The downward biasing can beachieved with a spring, or gas inputs can be provided to both raise andlower the screw.

The invention further includes a method for dispensing that includessteps of axially moving an augering screw away from a valve seat,rotating the screw to dispense liquid, and axially moving the augeringscrew into the valve seat to seal the liquid in the augering chamberfrom the nozzle. Prior to these steps, a step of adjusting the amount bywhich the shaft is raised can be performed. These steps of axiallymoving can be done by raising the screw away from the nozzle to allowflow, and lowering the screw toward the nozzle to seal the chamber, orvice versa.

In workable alternative embodiments, a liquid dispenser has a needlevalve in which a vertically oriented needle is raised and lowered toform a seal with the valve seat. Extending at an acute angle relative tothe vertical needle is a cartridge with an augering chamber forreceiving liquid and an augering screw. In this embodiment, the augeringscrew meters the liquid while the needle valve is separately actuable toopen and close the valve. While such a device is a workable embodiment,it is less desirable than using the screw to perform both the meteringand the valving functions because it is much less compact. Anotherseparately controllable valve can be provided, such as a rotatable ballvalve having an opening that can be aligned with the passage between theaugering chanter and the outlet of the nozzle.

A single augering screw serving as both a valve and an auger allows thedispensing and valving functions to be performed efficiently andcompactly. By using an improved design for the augering screw, thevolume of liquid that can be dispensed per revolution is increased, andleakage is further reduced because there is less change in pressurearound the screw. Other features and advantages will become apparentfrom the following detailed description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(c) are cross-sectional views of three operational states ofa pump assembly according to a first embodiment of the presentinvention; namely, a closed state, an open state, and a transitionalstate, respectively.

FIG. 2 is a close-up cross-sectional view of a cartridge in the pumpassembly of FIGS. 1(a)-1(c).

FIG. 3 is a cross-sectional views of a pump assembly according to -asecond embodiment of the present invention.

FIG. 4 is a cross-sectional view of a third embodiment of the presentinvention with an adjustable micrometer.

FIGS. 5(a)-5(b) are cross-sectional view of a fourth embodiment of thepresent invention with a valve arrangement that is different from thatin the first embodiment.

FIG. 6 is a block diagram of a control system according to the presentinvention.

FIGS. 7(a), 7(b), and 7(c) are cross-sectional views of 30 a prior artaugering screw, and first and second embodiments of an augering screwaccording to the present invention, respectively.

FIG. 8 is a cross-sectional view of an embodiment of a dispenser with anaugering screw and a needle valve.

DETAILED DESCRIPTION

The present invention improves the accuracy of liquid dispensing bypreventing undesired leakage through a nozzle while providing a compactdesign and avoiding excess wear on a motor. The liquid dispensing systemof the present invention has an angering screw that augers liquid andalso is axially movable into and out of a valve seat. The screw thusserves as both a metering device and a valve. When liquid is to bedispensed, the augering screw is moved away from the valve seat to opena passage between an augering chamber and an outlet of a nozzle; afterliquid is dispensed, the augering screw is moved into the valve seat tosubstantially seal the chamber from the outlet to prevent leaking. Thescrew also preferably has improved contours in the threading and anincreased number of threads for increasing the volume and the filling ofliquid around the screw, and thus to provide more liquid moreefficiently with less turning.

Referring to FIGS. 1(a)-1(c) and FIG. 2, a pump assembly 10 has acartridge 11 that receives liquid through a liquid inlet 12, andcontrollably directs a small amount of the liquid through an outlet 15in a nozzle 14. The nozzle rests in a threaded nut 17 that is screwed tocartridge 11. Cartridge 11 has a housing 20 with a bore that receives anaugering screw 16. An augering chamber 18 is not explicitly shown as anopen area, but is a region defined between screw 16 and housing 20.Screw 16 is controllably rotated to direct liquid downwardly throughoutlet 15 of nozzle 14 to dispense a region 21 of liquid onto a medium22, such as a printed circuit board.

The liquid is preferably a high viscosity liquid (more than about 10centipoise) that is dispensed over a semiconductor device to encapsulateit, or around a chip to underfill it. The liquid is preferably kept in acontainer (not shown) under constant pressure of about 30-40 PSI. Whendispensing to encapsulate, pump assembly 10 is moved in an x-y planethat is generally parallel to the chip, and is raised and lowered alonga z-axis (perpendicular to the x-y plane) with stepper motors (notshown). Depending on how the chip is mounted, it may be desirable tofirst create a dam around the border with a very high viscosity liquid,about 10⁶ centipoise, before filling in the region enclosed by the damwith a high viscosity liquid, about 10⁵ centipoise. These two dispensingsteps would typically be done with separate dispensers mountedside-by-side. In some cases it may not be necessary to form a dam; e.g.,if the chip is set in a recessed area of the circuit board. Whenunderfilling, liquid is dispensed around the chip and is drawn under thechip to fill a vertical gap between the chip and the circuit board.

Referring particularly to FIG. 2, augering chamber 18 is sealed off atits uppermost end with an O-ring 24 under compression from a spring 32.Spring 32 extends between a lower side of an upper first washer 30 andan upper side of a lower second washer 34, which in turn has a lowerside that contacts the O-ring. First washer 30 is held down with a nut28 that is threaded into an upper end of cartridge housing 20.

Referring again to FIGS. 1(a)-1(c), screw 16 is rotatably coupled to anddriven by a motor 40. The motor turns an output shaft 42 that isconnected to a flexible first coupling 44, such as a metal bellows,which is connected to drive shaft 46 with a spline (not shown) thatallows relative axial movement but maintains rigid rotatable movement.Screw 16 is preferably formed by cutting threads at the lower end ofshaft 46, and thus shaft 46 and screw 16 are one integral piece. Thesethreads extend from the lower end of the shaft to a position slightlyabove liquid inlet 12. A collar serving as a shoulder 48 is mounted overwasher 30 and is held in place with a radial set screw 49. Thus, outputshaft 42, drive shaft 46, screw 16, first coupling 44, and shoulder 48are all axially aligned, and all move together rotatably; and driveshaft 46, shoulder 48, and screw 16 move together axially.

These axial components are housed compactly in dispenser 10 with a lowerhousing 60 that surrounds cartridge housing 20, a lower lid 62 overlower housing 60, an upper housing 64 over lower lid 62, and an upperlid 66 over upper housing 64. Motor 40 is mounted over upper lid 66 sothat output shaft 46 extends through an opening in upper lid 66.

Motor 40 can be a stepper motor that is stopped and started for shortperiods of time to rotate the screw. Alternatively, screw 16 can berotatably coupled to a motor via a clutch mechanism. The design with aclutch requires the clutch as an additional component, but has anadvantage in that the motor can be operated continuously, rather thanstopped and started frequently. For applications with more continuousflow than dispensing dots, such as using an encapsulating liquid tocover an area, a stepper motor without a clutch may be more desirable.

As shown here (and as in FIGS. 1(b) and 1(c)), screw 16 has a lower end70 in an upper position such that it is spaced from a valve seat 72. Asshown here, valve seat 72 is formed in nozzle 14, although a valve seatcould be formed in a separate piece over the nozzle or in other ways(see FIGS. 5(a) and 5(b)). In this upper position, a passageway 74 isopened between chamber 18 and outlet 15 of nozzle 14. In a lowerposition, the lower end of the screw is in a position as indicated bydashed line 70 a (FIG. 2) in which the lower end rests in valve seat 72.Augering screw 16 thus seals chamber 18 from outlet 15 sufficiently tosubstantially prevent liquid from flowing between chamber 18 and outlet15 of nozzle 14. While lower end 70 of augering screw 16 is shownrounded and the valve seat conical, lower end 70 and valve seat 72 maytake any shape that allows them to conform with each other tosubstantially seal against the flow of the liquid. The seal need only besufficient for the particular liquid that is used, and that liquid maybe very viscous.

The ability of augering screw 16 to be axially moved in a controllablemanner results from an assembly, mounted over lower lid 62, andincluding a retainer clip 80 that is rigidly coupled about drive shaft46; a piston 82 that moves axially relative to drive shaft 46; a firstspring 84 that biases piston 82 downwardly; a second spring. 86 thatbiases shoulder 48 downwardly; a pneumatic inlet 88 for providing airthat drives piston 82 upwardly; and an exhaust vent 90.

Piston 82 and spring 84 are housed in and surrounded by a cup-shapedbody 96 that is fixed relative to the housing and surrounded by upperhousing 64. Body 96 extends around piston 82 and drive shaft 46, and hasa lower central bore that is slightly larger than the diameter of piston82 to allow piston 82 to move axially relative to body 96. An annularcap 94, which has a central bore sized to receive piston 82 for relativeaxial movement is rigidly mounted over cup-shaped body 96 and is alsosurrounded by upper housing 64.

Piston 82 has an axially elongated portion 100 with a central borethrough which drive shaft 46 extends, and thus piston 82 is coaxial withdrive shaft 46. Piston 82 and drive shaft 46 are spaced slightly overmost of the common length, but touch at each end for stability. Therethey touch, a slippery bearing surface allows relative rotatablemovement. Such a slippery surface can be provided by using a slipperyfinishing coating, such as a nickel and PTFE-based coating.

At the axial midpoint of elongated portion 100 of piston 82, an integralannular shelf 102 extends radially outwardly from elongated portion 100.At the outer end of shelf 102 are an upper lip 103 and a lower lip 105that define in the shelf an upper annular groove 104 and a lower annulargroove 106, respectively. Drive shaft 46 and cup-shaped body 96 definean annular region 98 within which first spring 84 and shelf 102 ofpiston 82 move vertically. First spring 84 is compressed verticallybetween a lower side of cap 94 and piston 82 at its upper annular groove104 to bias piston 82 downward.

Pneumatic inlet 88 has a horizontal inward branch and a vertical upwardbranch for directing air upwardly into region- 98 underneath lowerannular groove 106. Exhaust vent 90 extends vertically through cap 94 toallow air to escape region 98 to a region surrounded by upper housing64. Upper housing 64 has openings in its sides, preferably one largeopening on each of two opposite sides, to allow air to escape and toprovide useful access to portions of the dispensers by the user.

At its uppermost end, elongated portion 100 of piston is 82 abuts, butis not rigidly connected to, retainer clip 80. Clip 80 is annular with across-sectional area similar to that of elongated portion 100. A bearingsurface where piston 82 meets clip 80 is slippery so that piston 82 doesnot rotate with retainer clip 80 and drive shaft 46 when piston 82 abutsclip 80.

The different positions of drive shaft 46 and screw 16 are illustratedin FIGS. 1(a)-l(c), which show three respective positions: anon-dispensing position, a dispensing position, and a representation ofa transitional position between the dispensing and non-dispensingpositions.

Referring particularly to the non-dispensing position shown in FIG.1(a), pneumatic inlet 88 is not receiving air pressure, so first spring84 biases piston 82 downwardly so that piston 82 bottoms out so thatlower lip 105 contacts cup-shaped body 96. A horizontally disposedexhaust vent 89 allows air to be vented from a region between piston 82and nut 28. Second spring 86, between nut 28 (which is fixed relative tohousing 20) and shoulder 48, biases shoulder 48 downwardly, thus causinglower end 70 of augering screw 16 to be in valve seat 72. In thisposition, lower end 70 seals augering chamber 18 sufficiently tosubstantially prevent any viscous liquid from passing from augeringchamber 18 to outlet 15 of nozzle 14.

Referring to FIG. 1(b), when it is desired for pump s assembly 10 todispense liquid, air is provided under pressure from a pneumatic drivethrough pneumatic inlet 88 as indicated by flow arrows 110. This airflow forces piston 82 upwardly, thus urging retainer clip 80 upwardly.Because clip 80 is rigidly connected to drive shaft 46, from which screw16 is integrally formed, the upward movement of piston 82 causes driveshaft 46 (and hence screw 16) to be raised. Second spring 86, whichbiases shoulder 48 downwardly, is also compressed by the upward movementof piston 82 and clip 80. The upward force provided by the air throughpneumatic inlet 88 should therefore be sufficient to counteract thecombined downward biasing forces of first spring 84 and second spring86. When lower end 70 of screw 16 is raised away from valve seat 72,passageway 74 is opened between augering chamber 18 and nozzle 14. Screw16 can now be rotated by motor 40 to cause liquid to be dispensedthrough nozzle 14.

FIG. 1(c) illustrates a transitional position between the open positionof FIG. 1(b) and the closed position of FIG. 1(a). When the air pressureprovided through air inlet 88 stops, the air vents out of region 110through inlet 90, and first spring 84 again biases piston 82 downward.Piston 82 is not connected to retainer clip 80, 50 it does not pullretainer clip 80 downwardly. But because piston 82 is no longer biasingdrive shaft 46 and shoulder 48 upwardly, second spring 86 between nut 28and shoulder 48 can again bias shoulder 48 downwardly, thus pushingdownwardly on drive shaft 46. This downward movement returns clip 80 toits position in FIG. 1(a) in which it has a top surface that isgenerally co-planar with a top surface of cap 94, and returns lower end70 of augering screw 16 to valve seat 72 to block passage 74 (FIG. 1(b))between augering chamber 18 and nozzle 14 to prevent any leakage ofliquid.

FIG. 1(c) illustrates the interaction between first spring 84 and secondspring 86 only for a moment after the air inlet stops; in actual use,however, as piston 82 moves downwardly, drive shaft 46 and clip 80 willfollow piston 82 because of the biasing of second spring 86 so thatthere would be at most a slight gap between retainer clip 80 and piston82.

Referring to FIG. 3, in a second embodiment of the present invention, aspring for biasing piston 82 downwardly is omitted, and vent 90 in FIGS.1(a)-1(c) is replaced with a second pneumatic air inlet 120. In thisembodiment, pneumatic air inlet 88 is used to receive air to drivepiston 82 and clip 80 upwardly as shown in FIG. 1(b), and then air isintroduced through inlet 120 to assist in driving piston 82 downwardly.The forced air provided at pneumatic input 120 assists spring 86 inreturning drive shaft 46 and augering screw 16 back to the lowerposition in which lower end 70 rests in valve seat 72. In thisembodiment, pneumatic inlets 88 and 120 also serve as pneumatic outletswhen air is introduced through the other of the two. The pneumaticdrives for the two inlets can be different or combined.

Referring to FIG. 4, in a third embodiment, the pump assembly furtherincludes a micrometer 124 that allows the amount of vertical movement ofthe drive shaft to be controlled by a user. A piston 128 is threadedover a small area at its top end, and micrometer 124 has a threaded boreso that it is screwed over that area of piston 128. Micrometer 124surrounds, but is not rigidly connected to, drive shaft 126. At its topsurface, a non-metal insert 138 is snapped into micrometer 124. At itsouter diameter, micrometer 124 has gear teeth that mesh with teeth of anelongated gear 130. Gear 130 is rigidly connected to a knurled knob 13250 that gear 130 can be manually rotated by rotating knob 132. Knob 132is preferably easily manually accessible to the user. Gear 130 hassufficient axial length and an appropriate bearing surface to allowmicrometer 124 to travel axially relative to gear 130.

In FIG. 4, micrometer 124 is shown at a lowest position, about 0.1inches from a retainer clip 134 that is rigidly connected to drive shaft126 (note that the dimensions here are exaggerated and not to scale). Inthis position, as piston 128 is raised, insert 138 barely touches 10clip 134, and therefore shaft 126 is not raised.

As knob 132 is turned as indicated by arrow 136 a, micrometer 124 movesaxially upwardly relative to gear 130, shaft 126, piston 128, and clip134 as indicated by arrows 136 b. At its maximum upward position, insert138 contacts (or nearly contacts) clip 134. At this position, whenpiston 128 is raised (e.g., under air pressure), drive shaft 126 israised to its maximum amount. The micrometer can alternatively beconstructed as a large knurled ring that is manually accessible. In thiscase, the separate knurled knob 132 and elongated gear 130 can beeliminated, thus reducing the number of parts.

The position of knob 132 is preferably fixed with a resistive clamp (notshown) that provides frictional forces to keep the knob in position.Alternatively, a collar can be put under the micrometer and held with aset screw. With the micrometer, therefore the height by which the shaftmoves can be adjusted from about 0-0.1 inches. The precise level of themicrometer is set by the user, and generally would be based on thevolume of liquid to be dispensed, and the viscosity of the liquid.Generally, the micrometer will be higher (closer to the clip), thusraising the shaft higher, if the volume and/or viscosity is high.

Referring to FIGS. 5(a) and 5(b), in a fourth embodiment, the dispenserhas an annular valve seat formed in a cartridge housing. Screw 140 has areduced diameter lower end 142 that is threaded, and includes a nut 143screwed over lower end 142. The nut, which is preferably made of carbideor some other hand material, has a chamfered upper, outer edge 144.Screw 140 is mounted in cartridge housing 146, and thus defines anaugering chamber between screw 140 and housing 146. The lower part ofthe housing is shaped so that it creates an annular valve seat that isopened and closed by contact with edge 144 of nut 143. A nozzle 147 ispositioned under nut 143.

When the screw is in an upper position as shown in FIG. 5(a), the liquidis sealed within the chamber and cannot get to an outlet 148 of nozzle147. When the screw is lowered, as shown in FIG. 5(b), an annular flowpath is created from the annular chamber as shown by the arrows. Theliquid can thus flow through a passageway 149. When a dispensing cycleis completed, the screw is raised again into the valve seat as in FIG.5(a). This raising action is also beneficial because it has a suctioningeffect to help draw liquid upwardly into the augering chamber.

As shown in detail in the embodiment of FIGS. 1-3, the 20 screw in FIG.5 can be moved with pneumatic inputs in both directions, or the screwcan be biased either upwardly or downwardly with a spring that can beovercome with a pneumatic input in the opposite direction.

A block diagram of a control system for operating such 25 liquiddispensing systems as shown in FIGS. 1-5 is provided in FIG. 6. Acontroller 150, preferably including an appropriately programmedpersonal computer, provides to motor 40 drive signals that cause motor40 to start and stop, and provides to pneumatic drive 152 signals thatcause air to be provided through pneumatic inlet 88. Controller 150 alsocontrols other functions such as causing pressure to be provided on theliquid in a syringe and activating lead screw motors 160 that causedispenser 10 to be moved along three mutually orthogonal axes.

In the embodiment of FIG. 3 with two air inlets for moving the pistonupwardly and downwardly, controller 150 also controls movement of asecond pneumatic drive 156 (which may be combined with pneumatic drive152). In embodiments in which the motor is coupled to the screw througha clutch 160, controller 150 controls a signal provider 158 (typicallyprovided as a circuit board in the controller) that causes the signalpulses that activate the clutch.

Software for controlling such functions is generally known in the art,and can be adapted to implement the specific functions recited above.Other generally known software would also typically be provided forreceiving information about where and how liquid is to be dispensed, andfor establishing a routine based on the received information.

Referring again to FIGS. 1(a)-1(c), the present invention also includesa method for dispensing a liquid on a medium. According to this method,augering screw 16 is raised away from valve seat 72 and is rotated inorder to dispense liquid through outlet 15 of nozzle 14. Then anappropriate amount of liquid has been dispensed, augering screw 16 islowered so that it is in valve seat 72 and seals augering chamber 18from outlet 15 of nozzle 14. Preferably, augering screw 16 is raised bythe controller by providing a signal to the first pneumatic drive toraise the piston, and then providing a signal to a motor or to a clutchso that the motor causes the screw to turn and thus to dispense liquid.To stop dispensing, the signal to the motor or clutch, and the signal tothe pneumatic drive are changed; or a signal is provided to the secondpneumatic drive in the embodiment of FIG. 3.

In the embodiment of FIG. 5, the method includes lowering the screw toopen a passageway from the chamber, turning the screw to auger liquid,and then raising the screw to seal from the chamber.

Referring to FIGS. 7(a)-7(c), to further improve repeatability and toprevent air from being trapped in the corners of an augering screw withsquare contour between the threads, the screw has been redesigned fromprior screw designs. Referring to FIG. 7(a), a prior screw 180 that hasbeen used in liquid dispensers has threading with rectangularcross-sectional regions 182 between threads 184. The maximum diameter(including the threads) is about 0.2 inches, and regions 182 have adepth of about 0.01-0.03 inches and an axial length of about 0.045inches. While such a design appears to be desirable because it providesa large volume between threads for the liquid, it has been found thatthe liquid does not completely fill the regions and air gets in corners186 of regions 182. Because air compresses under force, when the forcestops, the air in the corners can expand and push outwardly on theliquid around screw 16, thus encouraging the liquid to leak through thenozzle.

Referring to FIG. 7(b), in a first embodiment of an augering screwaccording to the present invention, screw 188 has threads 190 withregions 192 between threads having a curved contour rather than a squarecontour. In this embodiment, the threads are angled at about 34° , themaximum diameter of the screw (thread to thread) is about 0.1-0.2inches, and the contour between threads has a radius of about 0.01-0.05inches.

Referring to FIG. 7(c), a screw 194 has a similar maximum diameter ofabout 0.1-0.2 inches, and radius of about 0.01-0.05 inches. The designof screw 194 has been further altered to enhance the quantity of liquidthat is dispensing per turn of the screw. The screw has a plurality ofhelical thread-defining channels that are evenly distributed about thecircumference of the screw, preferably two channels offset by 180° asshown.

While this curved contour allows less liquid to fill around the screwthan rectangular regions, the filling of the regions between the threadsis more complete, and therefore less air between the threads getscompressed and can expand.

When the liquid dispensers described above are used for dispensing aliquid over a semiconductor device for encapsulation or forunderfilling, the liquid can be provided in a continuous stream to coveran area, and the dispenser can be moved in a raster or spiral pattern ina horizontal plane with lead screw motors (FIG. 6). A dispenser asdescribed above is particularly appropriate for such an applicationbecause the application requires a high degree of accuracy.

Referring to FIG. 8, controlling the flow of liquid between an augeringchamber 200 and an outlet 202 of a nozzle 204 can also be accomplishedwith other types of valves. In this example, an augering screw 206 isprovided at an acute angle relative to the horizontal and vertical axesto provide liquid through an upper opening 208 at the top of nozzle 204.As discussed above, augering screw 206 is driven by a motor eitherdirectly or through a clutch to meter liquid through the nozzle. Inaddition to controlling screw 206 through a motor, the controller alsocontrols needle 210 to move it vertically to selectively seal and exposeopening 208 to prevent liquid from flowing or to allow liquid to flow.The valving function could be performed with some other type of valve,such as a ball valve, which has an off-center opening that can be movedinto and out of alignment with an opening in a nozzle.

The devices with a needle valve, a ball valve, or some other valve thatis independent of an augering screw are provided here as alternatives,but these alternatives are less desirable than the devices shown inFIGS. 1(a)-1(c), 3, 4, and 5, which allow both the metering function andthe valving function to be accomplished with a single screw, thusproviding better control over liquid flow, and allowing the dispenser tobe provided in a compact arrangement that need not be any larger involume than a comparable dispenser without such an axially movableaugering screw.

Having described preferred embodiments of the present invention, itshould be apparent that modifications can be made without departing fromthe scope of the claims appended below. For example, the use of theterms upward, downward, horizontal, and vertical are provided forrelative movement, but the dispenser could be disposed with otherorientations.

What is claimed is:
 1. An apparatus for dispensing small metered amountsof liquid onto a medium, the apparatus comprising: a housing; anelongated valve having a lower end and mounted in the housing, theelongated valve and housing defining a chamber for receiving liquid; anozzle fluidly coupled to said chamber for receiving liquid from saidchamber and having an outlet for dispensing the received liquid onto themedium; a valve seat between said chamber and said outlet of saidnozzle; and means for axially moving said elongated valve between asealing position in which said lower end of said elongated valve is insaid valve seat to substantially seal said chamber from said outlet ofsaid nozzle, and a dispensing position in which a fluid passage is openbetween said elongated valve and said outlet of said nozzle to allowmaterial to flow from the dispenser; wherein said elongated valve ispart of a drive shaft, and said moving means includes: a retainer cliprigidly coupled to said drive shaft; and a piston disposed around saiddrive shaft and positioned at least close to said retainer clip, saidpiston being axially movable relative to said drive shaft to contactsaid retainer clip for axially moving said retainer clip, said driveshaft, and said elongated valve.
 2. A method for dispensing smallmetered amounts of liquid from a dispenser, the dispenser having aninlet that receives liquid from a container, a nozzle from which thematerial is dispensed, a chamber, an elongated valve extending throughthe chamber to the nozzle, the method comprising the steps of: (a)moving the elongated valve from a sealing position in which theelongated valve is in a valve seat to create a seal to a dispensingposition; (b) forcing material to flow from the container through thehousing to the nozzle to dispense material from the dispenser while theelongated valve is in the raised position, wherein step (b) includesproviding a signal to a clutch to connect a motor and the elongatedvalve; and (c) moving the elongated valve from the dispensing positionto the sealing position to seal the chamber from the nozzle.